Rotary, push-pull combination switches have received widespread use within the automobile industry. Typically, the push-pull action of the switch operates the headlights, while the rotary action operates a dimmer switch which uses a rheostat to control the intensity of the instrument panel lights. The rotary action can also be used to control supplemental electrical circuits such as a dome light bypass switch which prevents door switches from energizing the dome light.
Combination automobile headlight switches are subject to several general design limitations. First, the switches should be compact to fit in the confined space allocated to them. Second, the switches should be uniform in size and design in order to fit within a wide variety of automobiles. Finally, the switches should be easy to fabricate at a low cost. The dimmer switch used to control instrument panel light intensity is subject to the additional limitation that it be capable of handling currents of up to 5 amps.
Typically, the dimmer switch portion of a combination headlight switch incorporates a rotatable, cemented resistor wire rheostat. Such a combination switch is disclosed in commonly assigned U.S. Pat. No. 4,827,241 to Riser et al. Cemented resistor wire rheostats comprise coils of resistor wire cemented together and a contactor which moves along the coils to vary the resistance of the rheostat. While cemented resistor wire rheostats are capable of reliable operation, they have several drawbacks. For example, their operational feel and sound quality can be affected by the assembly process or by the need to vary the number of coils and resistor wire diameter to provide the resistance required in a particular application. In addition, the number of components used to make the rheostats and the relatively wide dimensional tolerances of each part can lead to tolerance stack up when the switches are assembled. Moreover, the cemented wires in the rheostats are subject to breakage during assembly, leading to potential quality or reliability problems. Finally, the rheostats are subject to operational runout or wobble, which can lead to problems with operational stability. Other types of rheostats, including stationary, flat rheostat panels attached to heat sinks, are generally known. Commonly assigned U.S. Pat. No. 4,885,434 to Vultaggio et al. proposed the use of stationary flat panel rheostats in automotive headlight switches.
Combination switches used as headlight switches may also have a supplemental switch to control the dome light bypass circuit. One such switch uses a rotating rheostat with a projection which engages a dome light bypass switch externally mounted on the side of the headlight switch housing. The dome light bypass switch consists of an exposed arm extending longitudinally up the side and parallel to the shaft. This switch, however, may be too large for many applications, is susceptible to damage due to exposure of the external arm, and to tolerance stack up during fabrication.
U.S. Pat. No. 4,885,434 discloses a headlight switch with rotating arms mounted on the shaft and a stationary rheostat. A dome light bypass switch is internal to the headlight switch housing and functions by having one of the rotating arms engage a plunger which pushes open the dome light bypass switch. The plunger and dome light bypass switch are disposed on the stationary rheostat. Although this headlight switch solves the problems related to the size of such switches, it is difficult to fabricate due to the use of several leaf-type contacts which require extraordinary care during fabrication in order to prevent damage.
U.S. Pat. No. 4,827,241 suggests an alternative solution which uses a projection on a driver assembly to engage a dome light bypass switch which is sandwiched in line with the rotating components. As with the headlight switch disclosed in U.S. Pat. No. 4,885,434, the dome light bypass switch is internal to the headlight switch housing, which results in a compact switch. In addition, this headlight switch is very forgiving to tolerance stack up during fabrication. Unfortunately, the rotational components have to be small due to the sandwich arrangement and this leads to an overheating problem which limits the switch to low current applications (less than 5 Amp). The low current limitation makes this switch inadequate for the desired automotive use.